The extension of body composition studies to research problems in osteoporosis, renal failure, and the study of aging, requires new measurements, and exposes new challenges to the assumptions underlying the models on which traditional methods were based. The newer techniques: dual-photon absorptiometry (DPA), body electrical conductivity (TOBEC), and impedance measurement (BIA) are not only easier to perform than the traditional methods, but they offer measurements of primary and elemental facets of body composition which, even were they invasive or expensive, would represent powerful and significant additions to the field of research, particularly to the "newer domains" for body composition, which extend to the more clinical arenas of severe illness (and therefore severe distortions of body composition), and to the many facets of the study of aging, both normal and abnormal. The challenge which motivates this proposal developed in the process of designing a large scale study of risk stratification in surgical candidates over age 65. However, these new techniques appear in a setting in which a confusing array of methods, often giving unresolvably contradictory results, already exists, providing a wealth of data concerning body composition which must, if at all possible, be used as the basis for new work. By analogy, new languages are now available for application to an old culture, but these new languages must be interpreted so that texts written in the old languages can be understood: a "Rosetta stone" is needed, before either research laboratories or patients may fully benefit from the new technologies. At the core of the differences between the old and new technologies is the nature of the traditional models, and their intrinsic assumptions. We will evaluate the assumptions of constant density, hydration, and potassium content of the lean body mass, and will develop a translation table for these cornerstone measurements which is applicable to an expanded range of application, both in research and in clinical medicine. We propose to study 336 subjects in two years, 24 male and 24 female, in each of seven age groups from 21 to 90, using the new methods, as well as underwater weighing, body water, potassium, exchangeable sodium, and, in a subset of 30 subjects, total body neutron activation analysis. Appropriate statistical methods will be applied to model building and hypothesis testing from the data.